Portable communication devices, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers, and the like, are configured to communicate over wireless networks. Accordingly, each such portable communication device includes a transmitter (TX) and a receiver (RX), typically connected to a common antenna, for sending and receiving data and control signals over the wireless network. In order to use the common antenna, a duplexer may be used to electrically connect the common signal path to the output of the transmitter and to the input of the receiver, so that the transmitter is able to send signals on a transmit frequency and the receiver is able to receive signals on a different receive frequency with negligible interference between the transmit and receive signals.
A duplexer provides coupling, while preventing the transmit signal generated by the transmitter from being coupled from the common antenna back to the input of the receiver and overloading the receiver. Generally, the duplexer includes two band-pass filters having different passbands for filtering the transmit and receive signals, respectively, thus preventing or reducing interference between the transmit and receive signals. The filters are connected in parallel at the common antenna, and need to have sufficient rejection for the opposite band.
The duplexer has three ports. The first port is connected to the antenna, the second port is connected to the transmitter and the third port is connected to the receiver. Transmit and receive signals are assigned to different frequency bands, referred to as the transmit (uplink) frequency band and the receive (downlink) frequency band, respectively. However, the transmit and receive signals co-exist at the common antenna, as discussed above. Thus, for the transmission path, the duplexer suppresses all signals outside the transmit frequency band, and for the reception path, the duplexer suppresses all signals outside the receive frequency band. Hence, the duplexer includes two frequency selective radio frequency (RF) filters, one filter for the transmit frequency band and the other filter for receive frequency band. Both filters are electrically connected to the common antenna port. In order to prevent the impedance of one of the filters to degrade the antenna side impedance of the other filter, an additional matching circuit is included.
The transmitters and receivers may be implemented in various types of wireless network, according to different communication standards, such as universal mobile telecommunications system (UMTS), global system for mobile communication (GSM), personal communications services (PCS), digital cellular system (DCS), international mobile telecommunication (IMT), and enhanced data rates for GSM evolution (EDGE). The communication standards identify separate bands for transmitting and receiving signals. For example, UMTS Band 1 (IMT) provides an uplink frequency band of 1920 MHz-1980 MHz and a downlink frequency band of 2110 MHz-2170 MHz; UMTS Band 2 (PCS) provides an uplink frequency band of 1850 MHz-1910 MHz and a downlink frequency band of 1930 MHz-1990 MHz; UMTS Band 3 (DCS) provides an uplink frequency band of 1710 MHz-1785 MHz and a downlink frequency band of 1805 MHz-1880 MHz; UMTS Band 7 (IMT-E) provides an uplink frequency band of 2500 MHz-2570 MHz and a downlink frequency band of 2620 MHz-2690 MHz; and UMTS Band 8 (GMS-900) provides an uplink frequency band of 880 MHz-915 MHz and a downlink frequency band of 925 MHz-960 MHz. Accordingly, a duplexer operating in compliance with a UMTS standard would include a transmit filter having a passband within the corresponding uplink frequency band, and a receive filter having a passband within the corresponding downlink frequency band.
Demand for smaller, less expensive and more efficient portable communication devices is significant. Therefore, reducing size and weight of portable communication devices, as well as reducing fabrication costs and increasing product yield, are priorities. For example, there is demand for the filters of duplexers in portable communication devices to be smaller, to consume less power, to have improved performance characteristics (such as lower insertion loss and higher out-of-band attenuation), and to operate at higher frequencies. Such duplexers may include resonators for filtering the transmit and receive signals, such as a thin film bulk acoustic resonators (FBARs) and/or bulk acoustic wave (BAW) resonators. Further, duplexers have been developed that include the FBAR and/or BAW resonator transmit and receive filters on a single chip, further reducing size and weight. However, interference between the transmit and receive filters further increases due to the single-chip topology, e.g., resulting in undesirable magnetic coupling and capacitive coupling (i.e., parasitic capacitance or electrical induction) between the transmit and receive filters.